The term monochromator covers a broad class of devices designed to produce one or more monochromatic components from a broadband input signal. An optical monochromator selects separate spectral components from a broadband optical signal and enables the reliable registration of these separate components by means of a detector. Monochromators may also be used to recombine spectral components into a broadband signal. The most commonly known monochromators exploit the dispersion or interference properties of light propagating through a material or space.
Dispersion occurs due to the dependence of the refractive index of a material on the wavelength of light propagating through it. When light is incident on the interface between two media with different refractive indices, spectral components will be refracted in different ways depending on their wavelength in accordance with Snell's law. The different wavelengths contained in a broadband signal can thus be separated spatially by propagation through dispersive material. This effect is used in the design of dispersion prisms.
The approach based on the phenomenon of interference is used in such devices as diffraction gratings and arrayed waveguide gratings (AWG). In these devices an interference pattern is created as a result of the different phase shift undergone by the various wavelength components in an optical signal. By making the interference pattern selectively constructive or destructive, specific wavelength components of the broadband optical signal can be made to propagate in different areas of space in the case of diffraction gratings or in different arms of a multipath waveguide in the case of AWGs.
Devices of the kind described above that spectrally resolve broadband optical signals find application in a range of areas, such as in the fields of spectroscopy, signal processing and WDM optical networking, to name but a few. For example, dispersion prisms and diffractive gratings are commonly used for optical spectrum analysers, while AWGs are key elements in WDM multiplexers and demultiplexers.
However, complex solutions are required when using dispersion prisms and diffraction gratings in optical spectrum analysers. In order to obtain good wavelength resolution the light paths must be long. This implies the use of either a large bulk element, i.e. a prism, or a complex cascaded system of diffractive gratings. Both solutions are relatively complex mechanically and have high tolerance requirements. They commonly also have multiple moving parts and consequently require careful handling and maintenance.
An essential requirement of AWGs is the precise alignment of many wavelengths in different light paths. This alignment, which provides the necessary phase shift for the different wavelengths, is also subject to strict tolerance requirements if it is to function correctly with a change in ambient temperature. Additionally, since the input optical signal is split into multiple wavelengths simultaneously, AWGs suffer considerable insertion loss.